[ci] Removed python version 3.6 #123
[ci] Removed python version 3.6 #123

Removed python version 3.6,
version 3.8 and 3.9 were already
there, tests worked fine.

Fixes #123

openwisp/django-x509,openwisp/django-x509,openwisp/django-x509,openwisp/django-x509